Radiation detectors can be used to measure energy spectrum of rays such as X-RAYs or γ-rays, and thus are one of the major approaches in nuclide recognition. The radiation detectors have been widely used for detection of radioactive substances in, e.g., nuclear radiation protection, nuclear security check, environment protection, and national security, etc. Currently, the radiation detectors can be generally categorized into two types, including scintillator detectors represented by a NaI (TI) detector and semiconductor radiation detectors represented by a high-purity Ge (HPGe) detector.
The scintillator detectors have advantages such as low price and simple manufacturing process. On-site portable γ-spectrometers are typically implemented by the scintillator detectors, e.g., the NaI or CsI detectors. However, the scintillator detectors have a low energy resolution and thus cannot meet measurement requirements of fine structures of complex spectrums. The semiconductor HPGe radiation detectors have a high energy resolution. However, most of the semiconductor HPGe radiation detectors need to be reserved or used in liquid N2 at about 77 K and thus need low-temperature containers and vacuum chambers, which will increase a total size of the detectors. Furthermore, the liquid N2 needs to be refilled frequently, which is inconvenient for fieldwork and accordingly limits the application of such detectors.
Compound semiconductor radiation detectors have many advantages such as high energy resolution and high detection efficiency. Also, they are small in size and portable, and can operate at room temperature. The semiconductor radiation detectors have been widely used in, e.g., environment monitoring, nuclear medical science, industrial nondestructive testing, security check, nuclear weapon penetration, aviation, aerospace, astrophysics, and high-energy physics, etc. Recently, extensive research has been done on various semiconductor materials including Ge, HgI2, GaAs, TiBr, CdTe, CdZnTe, CdSe, GaP, HgS, PbI2, and AlSb, etc. Research results show that the CdZnTe has an excellent performance at the room temperature and is a promising material for manufacturing the semiconductor radiation detectors.
The semiconductor radiation detector, e.g., one made of CdZnTe, may have a relatively high atomic number (48, 30, 52) and a large density (6 g/cm3) compared to the NaI scintillator detector, which enables a high detection efficiency of high-energy γ and X-RAYs. The CdZnTe has a bandwidth of about 1.5˜2.2 eV and has a good performance at the room temperature. Furthermore, CdZnTe crystal of high quality has a substantially stable performance within a large range of temperature. In the CdZnTe, it consumes about 4.6 eV to generate a pair of charge carriers, while in the NaI scintillator, the energy to be consumed is about 100 eV. As a result, statistical fluctuation of the number of charge carriers induced by rays in the CdZnTe is smaller than that in the NaI scintillator. Furthermore, the CdZnTe has a resistance above 1010Ω, which ensures low leakage current noise, thereby improving the energy resolution.
However, the carriers in the CdZnTe crystal have a short drifting length Lh because cavities in the CdZnTe crystal have a short lifetime and low mobility, resulting in a small value of μhτh. Carriers in different locations of the semiconductor crystal may contribute differently to a pulse amplitude. As a result, the energy resolution of the semiconductor radiation detector may deteriorate.
This type of semiconductor radiation detector is often designed to have an electrode structure with sensitivity to a single polarity in order to improve the energy resolution thereof. Charges generated from reaction between the rays and the crystal will move toward two opposite electrodes under an electric field produced by a specially-designed electrode structure. In particular, electrons will move toward an anode while cavities will move toward a cathode. The electric field has a low intensity in locations far away from the electrode so that cavities moving at a low speed are easy to be trapped. In contrast, the electrons, which move at a high speed and are thus not easy to be trapped, are finally collected by the electrode. In this way, the detector sensitive to the single polarity can be implemented. Such a detector can alleviate negative effects on the energy resolution due to the low migration speed of the cavities. However, the electrons may be trapped because of crystal defects, causing fluctuations in signal amplitudes at the collection electrode generated by charges produced in different locations of the semiconductor crystal. This may also deteriorate the energy resolution of the radiation detector.
Moreover, radiation detectors having a coaxial structure comprise a hole in a center of the crystal and the electrodes are arranged in the hole. However, it is difficult to drill the hole in the crystal and mechanical damages may be caused in fragile crystals. Such damages may introduce defects near the electrodes, thereby degrading efficiency of signal collection.
Therefore, it is desired to improve the electrode design so as to enhance the energy resolution and detection efficiency of the radiation detector.